This invention relates to electronic assembly technology and more specifically to solder bump interconnections for mounting IC chips and the like on interconnection substrates like silicon, ceramic, or printed circuit boards.
Solder bump interconnection techniques for both electrically contacting component packages and mounting them on interconnection substrates such as printed circuit boards has become widely used in the manufacture of electronic devices. Interconnection substrates includes several forms of electronic device supports including, e.g., silicon and ceramic. For convenience reference to such supports herein will be to printed wiring boards as a generic term.
State of the art component packages are small and lightweight and can be surface mounted to printed circuit boards using fine patterns of solder bumps. Typically, bumps or pads are formed on both the printed wiring board and the component package in mirror arrays that mate when the component package is properly placed. Assembly is completed by applying heat to melt the solder and form the solder bond and interconnection. This technique is used in flip-chip technology where the surface of the IC chip in the component package is provided with bonding pads or bumps and the chip is mounted upside down on the printed wiring board.
The solder bumps are formed on arrays of I/O contact pads prior to assembly. To facilitate localized or selective application of solder to the array of contact pads the surface of the pads should be solder wettable. The metal interconnection pattern typically used for integrated circuits boards or cards is aluminum. While techniques for soldering directly to aluminum have been tried, it is widely accepted that aluminum is not a desirable material to solder. Consequently the practice in the industry is to apply a metal coating on the aluminum contact pads, and apply the solder bump or pad to the coating. This coating is referred to as under bump metallization (UBM).
The metal or metals used in UBM technology must adhere well to aluminum, be wettable by typical tin solder formulations, and be highly conductive. A structure meeting these requirements is a composite of chromium and copper. Chromium is deposited first, to adhere to the aluminum, and copper is applied over the chromium to provide a solder wettable surface. Chromium is known to adhere well to a variety of materials, organic as well as inorganic. Accordingly it adheres well to dielectric materials, e.g. SiO2, SINCAPS, polyimide, etc., commonly used in IC processing, as well as to metals such as copper and aluminum. However, solder alloys dissolve copper and de-wet from chromium. Therefore, a thin layer of copper directly on chromium will dissolve into the molten solder and then the solder will de-wet from the chromium layer. To insure interface integrity between the solder and the UBM, a composite or alloy layer of chromium and copper is typically used between the chromium and copper layers.
While this UBM coating works well, and is used successfully in the industry, it is a multilayer coating which adds complexity and cost to the IC packaging operation. It also adds a new metallurgy (chromium) to the system. It would be desirable from the standpoint of compatibility and process simplicity, if the materials used in the UBM coating were the same as the base materials being soldered, i.e. copper and aluminum.
We have developed a new and simpler UBM that uses only aluminum and copper. A thick aluminum layer is provided on the aluminum bonding site of the IC, and a relatively thick copper layer is adhered directly to the aluminum layer. The bonding of copper directly to aluminum, while difficult or impractical to effect by conventional techniques, is easily accomplished by sputter deposition if the aluminum surface is properly prepared. The copper layer can then be soldered to the next interconnect level by conventional means. The new UBM can be used in any interconnection arrangement including surface mount, and is ideally suited for flip-chip assembly.